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## Calculating density

• Mattski
• Topic Author

### Mattski created the topic: Calculating density

Hi can someone work out this calculating density height equation for me? Having problems understanding it.

Elevation=1800ft
QNH=1027hpa
Temp=+26c

• Stuart Tait

• Dobbo

### Dobbo replied the topic: Calculating density

Afternoon Stuart,

Looking at the density altitude equation, PA is 1380ft which could be rounded to 1500ft at +26 degrees C at indicated
So if MSL is +15 degrees C and we assume +1 degree decrease in temp per 500ft of altitude, so 1500ft should be +12 deg C
Would that make it a +14 degree Isa day giving us a DA ( 14 x 120 = 1680) + 1380 = 3080ft
Am I doing the calculations wrong?
I'm doing some revision as I'm getting closer to my chopper flight exam.

Regards:

Dobbo

• Posts: 42

### SJM replied the topic: Calculating density

Good catch Dobbo - in Stuart's calculation of ISA temperature he may have accidentally used '26' rather than '15'.
Here is the corrected working out in extended form

Question Data
Altitude: 1800 ft
QNH: 1027 hPa
Temperature: 26 degrees c

Pressure Height = altitude + 30(1013-QNH) = 1800 + 30(1013-1027) = 1800 + 30(-14) = 1380
ISA Temp = 15 - 2(1500/1000) = 15 - 3 = +12
ISA Deviation = OAT - ISA Temp = 26 - 12 = plus 14
Density height = Pressure Height + 120(ISA Deviation) = 1380 - 120(14) = 3060 feet
---
You can also use a flight computer - just work out the pressure height and set the temperature against pressure height and read the value in the density altitude window. Note it might read 3000 ft - near enough, good enough

• Stuart Tait

### Stuart Tait replied the topic: Calculating density

Thanks for that must have had my brain in neutral